Artificial structure and method of preparing same



Patented Jan. 9, 1940 UNITED STATES.

Anni-"1on1.

STRUCTURE AND OF PREPARING SAME Ernst Freund and Friedrich Deutsch, Vienna, Austria, assignors to E. I. du Pont de Nemours &- Company, Wilmington, DeL, a corporation of Delaware .No Drawing. Application May 25, 1937, No. 144,760. In Austria June 4, 193.6

8 Claims. (01. 91- 94) This invention relates to the production of articles having highly lustrous, mirror-like surfaces and particularly relates to a process of producing the same.

One object of the invention is to produce an artificial structure characterized in having a relatively transparent, internal core and in having a highly lustrous, mirror-like surface consisting of a susbtantially smooth, uniform and continuous coating of lustrous, finely divided particles of a weakly electropositive metal.

' Another object of the invention is to produce an organic cellulose derivative film, thread, coatmg or the like having a highly lustrous, mirrorlike surface, comprising a relatively transparent,

interior portion of organic cellulose derivative and a substantially smooth, uniform and continuous coating of lustrous, finely divided particles of a weakly electropositive metal.

Another object of the invention is to provide a process whereby the above described articles may be obtained. Other. objects will become apparent in the specification that follows.

According to the invention, an article having a a highly lustrous, mirror-like surface is prepared bya process comprising incorporating a dissolved,

reducible salt ofa weakly electropositive metal into an article and producing a surface consisting substantially of a smooth, uniform, and continuous layer of lustrous, finely divided, metallic particles by subjecting the metallic salt contained in the article to the action of a reducing agent which will directly reduce the metal salt to metal without substantial formation of metal compounds of the said metal.

In the preferred embodiment of the invention, the previously dissolved, reducible, metallic salt is added to an organic cellulose derivative solution, and the solution converted into a film by a standard evaporative procedure. The resulting film is then immersed in an alkaline bath containing formaldehyde for a short time, withdrawn and dried. The final film exhibits on both sides a highly lustrous, mirror-like surface consisting of a smooth coating of finely divided, metallic particles.

The following examples will now be given to better illustrate the invention, and are not to be considered limitative.

Example I is added in such .a proportion that the solution contains about 6-7 parts of silver nitrate to 100 parts of cellulose acetate. The thus prepared, homogeneous solution is converted into a clear and transparent film or sheeting by most any standard method of dry castinfi and preferably in a manner yielding a film having a smooth surface. By treatment of the'resulting transparent film with an'aqueous reducing liquid containing 0.25% sodium carbonate, 0.2% formaldehyde and a mixture of ethyl alcohol and acetone,

there is obtained afilm having on both sides a highly lustrous, mirror-like surface consisting substantially of a smooth coating of finely divided silver. After careful washing and drying, the metallized film may, if desired, be subjected to burnishing or other after-treatments.

. Example II A sufficient quantity of silver nitrate, dissolv in 80% aqueous ethyl alcohol, is added to a I solution of ethyl cellulose in a binary solvent sisting substantially of a smooth coating oif finely divided silver. If desired, after careful washing, the metallized film may be treated with deacetylated chitin as described in U. 8. Patent No. 2,040,879. The dried, metallized film may be given a high degree of polish by burnishing orlike procedure.

' Example III An ac'etal type condensation product of formaldehyde with polyvinyl alcohol, such as is de-.

scribed in U. 8. Patents Nos. 2,036,092 and. 2,035,939, is dissolved ina binary solvent consisting of benzene and methanol, to give a polyvinyi formal concentration of 16% by weight. A solution of silver nitrate in 80% aqueous ethyl alcohol is added to the polyvinyl formal solution in suflicient amount so thatthe solution contains 6-8 parts of silver nitrate to 100 parts of the polyvinyl formal and the homogeneous solution is formed :into a smooth, clear and transparent film by a standard .dry casting procedure. The

7 clear film is passed by rollers through an aqueous liquid reduction bath containing 0.25% carbonate, 0.2% formaldehyde and 12% methanol at such a speed that the silver salt is completely reduced in the bath. The thus treated film has on both sides a highly lustrous, mirror-like surface consisting substantially of a smooth coating of finely divided, metallic silver. If desired, after passing through a washing bath, the metallised film may be led througha bath containing-deacetylated chitin and finally dried in some suitable way,. for example in a warm current of air. The so treated and dried film may be given a high polish by a procedure such as burnishing with rotating rollers.

In the above examples, base substances con sisting of cellulose acetate, ethyl cellulose and polyvinyl formal have been provided with a mirror-like, metallic surface. It will be recognized obviously that certain other base substances may be employed. Thus, cellulose acetate may be replaced with esters such as cellulose propionate, cellulose acetopropionate, celluloseacetobutyrate, cellulose acetate-propionate-butyrate, and cellulose acetostearate. Or, the ethyl cellulose may be replaced with propyl cellulose, butyl cellulose, benzyl cellulose and like cellulose ethers. Or, the polyvinyl formal may be replaced with condensation products of polyvinyl alcohol, with acetaldehyde, propionaldehyde, butyraldehyde and other suitable aldehydes. In this case the polyvinylalcohol reagent may or may not be partially esterified. Likewise, as the base may be used substances of the type such as polyvinyl alcohol, polyvinyl esters, interp'olymers of vinyl chloride and vinyl acetate, toluene sulfamide condensation products, and polymerized esters of acrylic and alkacrylic acids. 7

Although a metallized film represents the preferred embodiment of the invention, the metallized article may also take other forms and shapes. For instance, acetonic solutions containing cellulose acetate and silver nitrate may be spun through fine nozzles to form threads and fibers which may subsequently be metallized by treatment with reducing liquids, either in w a woven or unwoven condition. In like fashion articles such as caps, bands, ribbons, straws,

thick film, and plastics-may be secured which have a highly lustrous, mirror-like, metallic surface and a relatively transparent interior. The article of the invention may also be formed as a coating on various surfaces. Thus, a 20% solution of cellulose acetate in acetone and containing 6% silver nitrate on the basis of cellulose acetate may be sprayed as a lacquer on such surfaces as wood, metal, fabric or regenerated cellulose film which may or may not have been moistureproofed with the'usual nitrocellulose or like lacquers. After the lacquer hasdried, a metallic effect is produced by treatment of the coating with an alkaline formaldehyde solution.

The metallic salts of the invention are directly reducible to the metal and are in general compounds of the weakly electropositive metals such as gold, silver, platinum or copper. They are preferably metallic salts which are soluble in organic solvents containing a small amount of water. They may be simple metallic salts such as silver nitrate, silver acetate, auric chloride, and cupric nitrate or may be complex metallic saltssuch as silver ammonium nitrate, [Ag(NHa).2NOa], and silver ammonium bromide [Ag(NHa):Brl. Silver ammonium thiocyanate [Ag (N'Ha) aCNS] may also be employed, provided the salt is decomposed with 10% acetic acid prior to the reduction treatment.-

A concentration corresponding to 6-8 by weight of metallic salt to base substance such as cellulose acetate is preferably used in the process of the invention.. It will, of course, be recognized that smaller quantities like 4%, or larger quantities like 12% of metallic salt give satisfactory results if adjusted to factors such as concentration and characteristics of the base substance, the concentration and characteristics of the metallic salt, and the physical form I of the final product.

While formaldehyde is preferred, many other reducing agents which will directly reduce the metal salt to metal without substantial formation of metal compounds of the. said metal may be utilized with more or less advantage in the process of the invention. 'Ihus, among others,

acetaldehyde, hydrazine, hydroxylamine, tartaric acid, ultra-violet rays, and oxidizable salts of strongly electropositive metals like ferrous sulfate may serve the purpose of reducing the metallic salts to a finely divided metallic state. Agents, such as hydrogen sulfide, tend to form compounds with the metals and are not desirable reducing agents for the process of the incentration of 0.2% formaldehyde and 0.25% so-- dium carbonate is particularly good. A concentration of 0.2% formaldehyde and 0.05% sodium carbonate is still satisfactory, and a concentration of 2% formaldehyde and 5% sodium carbonate yields a metallic silverv having a blackishblue tinge. It may, however, be possible to use a higher concentration of reducing agent and alkaline-salt if the film or other article is contacted therewith for a very short period. Dependent on the particular reducing agent selected, the sodium carbonate may be replaced in whole or in part with ammonia, sodium hydroxide, triethanolamine, ethylene diamine, ethylamine or similar alkaline substances.

The alkaline substances may also be incorpo rated in the solutions containing the base substance and the reducible, metallic salt. Thus, for example, an alcoholicsolution of silver nitrate and triethanolamine may be stirred into an acetone solution of cellulose acetate and the -mixture molded to form films in the usual way and then treated with a reducing liquid. In this case. there is generally no need of alkaline substances being present in the reducing liquid during the production of the metal plating. Again,

ultra-violet light may be applied to the so treated film to produce the metal plating effect.

An alternate method of carrying out the proces of the invention consists of adding to an alkali free, or slightly acidic solution containing the base substance and the reducible, metallic salt a reducing agent that reacts only in alkaline media. For instance, an acetonic solution containing cellulose acetate and silver nitrate may be mixed with an acetonic solution of cellulose acetate containing hydroquinone and the resulting solution may be moulded into substantially clear films in the usual manner. Subsequently, the films may be metal plated simply by treatment with aqueous solutions of alkaline substances or with ultra-violet light.

It is preferred that the base material be in a swollen condition when the reduction treatment is carried out. Many materials, such as fairly highly saponified cellulose acetate or acetal type derivatives of polyvinyl alcohol, swell in water to a certain extent and for these, straight aqueous solutions of reducing agents may beused. If the base material does not swell much in water, it is advisable to add a swelling agent to the liquid containing the reducing liquid. For example, a film, produced-from a-solution containing cellulose triacetate and silver nitrate, is metallized by treatment at 15 C. with a solution of formaldehyde and sodium carbonate containing 70% ethyl alcohol; The aqueous alcoholic liquid swells the triacetate slightly. The swelling agent may also be added to the solution of the base material. For instance, ammonium 'thiocyanate may be added to a solution containing cellulose triacetate and a small quantity of auric chloride so that a film cast therefrom contains approximately 2% of ammonium thiocya- 1 ing bath. Non-solvents of this type, such as glycerin in the case of cellulose acetate, may also be added to the cellulosic or other solutions of material in conjunction with high boiling solvents. The use of non-solvents of this kind has the advantage that it often checks undesirable plasticity 'on the part of the product moulded from solutions containing high boiling solvents,

and still causes the product to swell to a certain extent even when subjected to a reduction treatexerted in polishing.

1 coating at the surface of the article.

ment in a non-swelling medium.

It will, of course, be evident that the solutions of theinvention "may also contain other substances such as dyes, pigments, and the like, provided they'are chemically inert to the metallicsalts or metals derived therefrom.

The metal plated articles of the invention may be treated with a transparent adhesive of suit able solubility simultaneously with or after metal plating. Likewise, the metallized article may be protected with a transparent lacquer coating.- The lacquers may be colored or fluorescent, and very distinctive effects may thus be obtained.

If desired, the metallized article may be given a high polish. Preferably, the metallized article is first polished dry with cotton, wool or the like, using only a slight pressure, after which the article may be wet slightly and a greater pressure It is distinctly preferred to have the metallic However, the metallic coating may also be formed by suitableadjustment of the conditions above described in such a fashion that the coating penetrates slightly into the relatively transparent interior, or that the coating is slightly sealed over by the base substance of the article.

As mentioned hereinbefore, the articles of the invention are characterized in having a substantially transparent interior which tends to augment the pleasant effect of the smooth, highly lustrous, mirror-like surface of the metallized article. The bright, mirror-like luster of the metallized article resembles that of a silver backing 01' a glass mirror when viewed through the glass and is rather surprising since the'same does not result on the outside surface it a metal such linery art.

I 3 as silver is precipitated on a film by dipping it in an ammoniacal silver nitrate solution to which formaldehyde is added or by coating the film with 'a lacquer containing mechanically suspended,

finely divided, metallic silver. The process of the inventionis also very efflcient inthe usage of the relatively expensive materials-gold, silver, platinum or the like. That is, the metal seems to come to the surface of the article during reduction of the metallic salt and therefore requires 'only a quantity of metallic salt sufficient to produce a substantially continuous coating of finely divided metal.

The articles of the invention find successful adaptation to many and varied uses. The metallized films may be fashioned into mirrors or refiectors, or may be cut with suitable dies into spangles to be used in securing decorative effects. The metallized films preferably roughened on one side may optionally be gilded by treatment with an auric chloride solution for use as cigarette tips. The fibers and threads of the invention may be woven into fabrics, draperies, theatrical costumes, theatrical curtains and the like. The metallic effect may be brought 0:. the fibers or threads prior, during or after fabrication. The metallized threads and fabrics made I therefrom may be used for hygienic hose or shoes.

Themetallized slit films, threads, straws and ribbons also find useful application in the milgiven an ornamental seal by means of the metallized caps of the invention, As a coating and lacquer, the article of the invention may be applied to wood, metal, regenerated cellulose film,

draperies, fabrics and the like.

Since many changes and modifications can be made from the above detailed description with- Bottles and like containers may be out departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited except as set forth in the appended claims.

We claim:

1. In a method for the production of lustrous, mirror-like articles, the steps comprising incorporating within a volatile solvent solution of an organic cellulose derivative taken from the group consisting of cellulose esters and cellulose ethers a directly reducible salt of a metal taken from the group consisting of copper, gold, silver and platinum, forming an article from said solution and reducing said salt to form a bright, lustrous metal within said article by means of a reducing agent which will directly reduce said salt without substantial formation of a metal compound of said metal. I

2. In a. method for the production of lustrous, mirror-like articles, the steps comprising incorporating within a volatile solvent solution of an organic cellulose ester a directly reducible salt of a metal .taken from the group consisting of copper, gold, silver and platinum, forming an article from said solution, and reducing said salt in the presence of an alkaline medium to form a bright, lustrous metal within said article by means of a reducing agent which will directly reduce said salt in the presence of said alkaline medium without substantial formation of a metal compound of said metal.

3. In a method for the production of lustrous, mirror-like articles, the steps comprising incorporating within a volatile solvent solution of an organic cellulose ester a directly reducible salt of a metal 1. km from the group consisting of copper, gold, silver and platinum, forming an agent which will directly reduce said salt in said alkaline solution without substantial formation of a metal compound of said metal, and polishing said article.

5. In a method for the production of lustrous, mirror-like articles, the steps comprising incorporating within a volatile solvent solution ofan organic cellulose derivative taken from the group consisting of cellulose esters and cellulose ethers a directly reducible salt of a metal taken from the group consisting of copper, gold, silver and platinum, forming an article from said solution, and reducing said salt in the presence of an alkaline medium with formaldehyde which will directly reduce said salt in the presence of said alkaline medium without substantial formation of a metal compound of said metal.

6. In a method for the production'of lustrous,

- mirror-like articles, the steps comprising incorporating within a volatile solvent solution of an organic cellulose derivative taken from the group amass:

consisting of cellulose esters and cellulose ethers a directly reducible salt of a metal taken from the group consisting of copper, gold, silver and platinum, forming an article from said solution, and treating said article with an alkaline solution containing formaldehyde which solution will directly reduce said salt without substantial formation of a metal compound of said metal.

. 7. In a method for the production of lustrous. mirror-like articles, the steps comprising incorporating within a volatile solvent solution of an organic cellulose derivative taken from the group consisting of cellulose-esters and cellulose ethers silver nitrate, forming an article from said solution, and treating said article with an alkaline solution containing formaldehyde which solution will directly reduce said salt without substantial formation of a metal compound of said metal.

8. In a method for the production of lustrous, mirror-like articles, the steps comprising incorporating within a volatile solvent solution of. an organic cellulose derivative taken from the group consisting of cellulose esters and cellulose ethers a directly reducible salt of a metal taken from the group consisting of; copper, gold, silver and platinum and an alkaline medium, forming a a cellulose derivative article from said solution, and treating said article with a solution of a reducing agent without substantial formation of a metal compound of said metal whereby to form a lustrous metal in the surface of said article.

ERNST FREUND. 1 r'ampmcn nrru'rscn. 

